The Multiple-Input Multiple-Out-put (referred to as MIMO) pre-coding technology is divided into pre-coding based on codebook and pre-coding not based on codebook. Comparatively speaking, the pre-coding not based on codebook can match the channel better and improve the link performance. The beamforming technology used in the Long Term Evolution (referred to as LTE) Time Division Duplexing (referred to as TDD) system is a pre-coding technique not based on codebook.
The beamforming estimates an airspace channel on which received signals propagate to obtain beamforming reception/transmission weighting value, and during a receiving/transmitting process, the input and output signals of each antenna in the array are weighted to achieve signal beamforming reception/transmission.
The LTE TDD mode uses the time division duplex mode, wherein uplink and downlink channels occupy the same frequency resource and have essential channel reciprocity, and it is very suitable for using the beamforming technique to improve the downlink performance. The basic operating procedure of downlink beamforming in the LTE TDD system is: first, an evolved Node B (eNodeB or eNB) estimates to obtain an uplink channel according to a Sounding Reference Signal (referred to as SRS) or Demodulation Reference signal (referred to as DMRS) transmitted in the uplink, and obtains the equivalent downlink channel according to the channel reciprocity, the eNB performs a downlink transmission beamforming vector estimation according to the equivalent downlink channel. Finally, the eNB uses the estimated beamforming vector to perform a downlink transmission, and performs the same beamforming on transmitted data and the UE-specific reference signals (UE-Specific RS). At the receiving end, a user equipment (referred to as UE) uses the UE-specific reference signal for channel estimation to obtain the equivalent channel, thus recovering the downlink transmitted data.
The Long Term Evolution advanced (LTE-A) R10 supports uplink MIMO (UL-MIMO) and downlink MIMO (DL MIMO), and supports up to four SRS port transmission in the uplink and up to eight antenna reception in the downlink at the UE side. When the number of the UE's reception antennas is greater than the number of SRS transmission ports, the eNB using the SRS or DMRS transmitted in the uplink can only obtain downlink radio channels between some but not all of the UE's reception antennas and the base station transmission antennas; in the LTE, when the number of UE's transmission antennas is 1, in order to better support the beamforming based on the channel reciprocity in the TDD mode, the method for switching the UE's transmission antennas can be used to support two antennas alternately transmitting the SRS. However, when the number of downlink MIMO configuration ports is greater than 2, the scheme cannot make the eNB have full access to the radio channels between the UE's antennas and the base station antennas. Because the existing schemes cannot obtain the downlink radio channels between all the UE's reception antennas and the base station transmission antennas, there will be an impact on the accuracy of using the channel reciprocity to calculate the beamforming weight value of the downlink MIMO, so that the system performance degrades.
In addition, when the number of antennas which can be used by the UE for uplink transmission is greater than the number of SRS transmission ports, only through SRS signals transmitted by the SRS transmission ports, the eNB is hard to obtain conditions of radio channels completely from all the UE's antennas to the eNB, and cannot transmit a physical uplink shared channel (PUSCH) with an optimal antenna or antenna group to obtain an optimal uplink transmission performance.